Margin test methods and circuits

ABSTRACT

Described are methods and circuits for margin testing digital receivers. These methods and circuits prevent margins from collapsing in response to erroneously received data, and can thus be used in receivers that employ historical data to reduce intersymbol interference (ISI). Some embodiments detect receive errors for input data streams of unknown patterns, and can thus be used for in-system margin testing. Such systems can be adapted to dynamically alter system parameters during device operation to maintain adequate margins despite fluctuations in the system noise environment due to e.g. temperature and supply-voltage changes. Also described are methods of plotting and interpreting filtered and unfiltered error data generated by the disclosed methods and circuits. Some embodiments filter error data to facilitate pattern-specific margin testing.

BACKGROUND

Signal distortion limits the sensitivity and bandwidth of anycommunication system. A form of distortion commonly referred to as“intersymbol interference” (ISI) is particularly problematic and ismanifested in the temporal spreading and consequent overlapping ofindividual pulses, or “symbols.” Severe ISI prevents receivers fromdistinguishing symbols and consequently disrupts the integrity ofreceived signals.

FIG. 1 (prior art) depicts a conventional receiver 100, which is usedhere to illustrate the ISI problem and a corresponding solution.Receiver 100 includes a data sampler 105 and a feedback circuit 110.Sampler 105 includes a differential amplifier 115 connected to adecision circuit 120. Decision circuit 120 periodically determines theprobable value of signal Din and, based on this determination, producesa corresponding output signal Dout.

Sampler 105 determines the probable value of signal Din by comparing theinput signal Din to a voltage reference Vref at a precise instant.Unfortunately, the effects of ISI depend partly on the transmitted datapattern, so the voltage level used to express a given logic level varieswith historical data patterns. For example, a series of logic zerosignals followed by a logic one signal produces different ISI effectsthan a series of alternating ones and zeroes. Feedback circuit 110addresses this problem using a technique known as Decision FeedbackEqualization (DFE), which produces a corrective feedback signal that isa function of received historical data patterns.

DFE feedback circuit 110 includes a shift register 125 connected to theinverting input of amplifier 115 via a resistor ladder circuit 130. Inoperation, receiver 100 receives a series of data symbols on an inputterminal Din, the non-inverting input terminal of amplifier 115. Theresulting output data Dout from sampler 105 is fed back to shiftregister 125, which stores the prior three output data bits. (As withother designations herein, Din and Dout refer to both signals and theircorresponding nodes; whether a given designation refers to a signal or anode will be clear from the context.)

Shift register 125 includes a number of delay elements, three flip-flopsD1-D3 in this example, that apply historical data bits to the referencevoltage side of the differential amplifier 115 via respective resistorsR1, R2, and R3. The value of each resistor is selected to provideappropriate weight for the expected effect of the correspondinghistorical bit. In this example, the value of resistor R3 is highrelative to the value of resistor R1 because the effect of the olderdata (D3) is assumed to be smaller than the effect of the newer data(D1). For the same reason, the resistance of resistor R2 is between theresistors R1 and R3. Receiver 100 includes a relatively simple DFEcircuit for ease of illustration: practical DFE circuits may sample moreor fewer historical data values. For a more detailed discussion of anumber of receivers and DFE circuits, see U.S. Pat. No. 6,493,394 toTamura et al., issued Dec. 10, 2002, which is incorporated herein byreference.

The importance of accurate data reception motivates receivermanufacturers to characterize carefully their system's ability totolerate ISI and other types of noise. One such test, a so-called“margin” test, explores the range of voltage and timing values for whicha given receiver will properly recover input data.

FIG. 2 depicts a fictional eye pattern 200 representing binary inputdata to a conventional receiver. Eye pattern 200 is graphed in twodimensions, voltage V and time T. The area of eye 205 represents a rangeof reference voltages and timing parameters within which the datarepresented by eye 205 will be captured. The degree to which the voltageV and time T of the sampling point can vary without introducing an erroris termed the “margin.”

FIGS. 3A through 3C depict three signal eyes 300, 305, and 310illustrating the effects of DFE on margins and margin testing. Referringfirst to FIG. 3A, eye 300 approximates the shape of eye 205 of FIG. 2and represents the margin of an illustrative receiver in the absence ofDFE. FIG. 3B represents the expanded margin of the same illustrativereceiver adapted to include DFE: the DFE reduces the receiver's ISI, andso extends the margins beyond the boundaries of eye 300. Increasing themargins advantageously reduces noise sensitivity and improves bit errorrates (BER).

In-system margin tests for a receiver are performed by monitoringreceiver output data (e.g., Dout in FIG. 1) while varying the referencevoltage and sample timing applied to the input waveform Din. Withreference to FIG. 2, such testing samples various combinations ofvoltage and time to probe the boundaries of eye 205, the boundariesbeing indicated when the output data does not match the input data.Margin tests thus require the receipt of erroneous data to identifysignal margins. Zerbe et al. detail a number of margin tests in “Methodand Apparatus for Evaluating and Optimizing a Signaling System,” U.S.patent application Ser. No. 09/776,550, which is incorporated herein byreference.

A particular difficulty arises when determining the margins ofDFE-equipped receivers. While feeding back prior data bits increases themargin (FIG. 3B), the effect is just the opposite if the feedback datais erroneous. Erroneous feedback emphasizes the ISI and consequentlyreduces the margin, as shown in FIG. 3C. The margin of a DFE-equippedreceiver thus collapses when a margin test begins to probe the limits ofthe test signal (e.g., the boundaries of eye 205). The incompatiblerequirements of erroneous data for the margin test and correct data forthe DFE thus impede margin testing. There is therefore a need forimproved means of margin testing DFE-equipped receivers.

The need for accurate margin testing is not limited to DFE-equippedreceivers. Errors in margin testing lead integrated-circuit (IC)designers to specify relatively large margins of error, or “guardbands,” to ensure that their circuits will perform as advertised.Unfortunately, the use of overly large margins reduces performance, anobvious disadvantage in an industry where performance is paramount.There is therefore a need for ever more precise methods and circuits foraccurately characterizing the margins of high-speed integrated circuits.

SUMMARY

The present disclosure is directed to methods and circuits for margintesting high-speed receivers. Some embodiments equipped with DecisionFeedback Equalization (DFE) or other forms of feedback that employhistorical data to reduce inter-symbol interference (ISI) perform margintests using a known input data stream. The receiver injects a copy ofthe known input data stream (i.e., the “expected data”) into thefeedback path irrespective of whether the receiver correctly interpretsthe input data. The margins are therefore maintained in the presence ofreceiver errors, allowing in-system margin tests to probe the marginboundaries without collapsing the margin. Receivers in accordance withsome embodiments include local sources of expected data.

Other embodiments do not rely on “expected data,” but can be margintested in the presence of any pattern of received data. Theseembodiments are particularly useful for in-system margin testing. Alsoimportant, such systems can be adapted to dynamically alter systemparameters during device operation to maintain adequate margins despitefluctuations in the system noise environment due to e.g. temperature andsupply-voltage changes.

Also described are methods of plotting and interpreting error datagenerated by the disclosed methods and circuits. One embodimentgenerates shmoo plots graphically depicting the results of margin tests.Some embodiments filter error data to facilitate pattern-specific margintesting.

This summary does not limit the invention, which is instead defined bythe allowed claims.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 (prior art) depicts a conventional digital receiver 100.

FIG. 2 depicts a fictional eye pattern 200 representing binary inputdata to a conventional receiver.

FIGS. 3A through 3C depict three signal eyes 300, 305, and 310illustrating the effects of DFE on margins and margin testing.

FIG. 4 depicts a communication system 400, including a conventionaltransmitter 402 connected to a DFE-equipped receiver 403 adapted inaccordance with one embodiment.

FIG. 5 depicts a DFE-equipped receiver 500 adapted in accordance with anembodiment to include improved means of margin testing.

FIG. 6 depicts a receiver 600 in accordance with another embodiment.

FIG. 7 depicts a receiver 700 in accordance with yet another embodiment.

FIG. 8 depicts an embodiment of a buffer 800, which may be used as oneof amplifiers 745 in weighting circuit 735 of FIG. 7.

FIG. 9 depicts a receiver 900 in accordance with another embodiment.

FIG. 10A depicts a receiver 1000, a simplified version of receiver 900of FIG. 9 used to illustrate margin mapping in accordance with oneembodiment.

FIG. 10B is a diagram illustrating the relationship between each ofsamplers 1005 and 1010 of FIG. 10A and a data eye 1030.

FIG. 10C depicts a shmoo plot 1050 graphically depicting an illustrativemargin test in accordance with one embodiment.

FIG. 11 details an embodiment of shmoo circuit 1025 of FIG. 10A.

FIG. 12 details a receiver 1200 in accordance with another embodimentadapted to accommodate margin shmooing.

FIG. 13 depicts a receiver 1300 that supports error filtering inaccordance with another embodiment.

DETAILED DESCRIPTION

FIG. 4 depicts a communication system 400, including a conventionaltransmitter 402 connected to a receiver (receive circuit) 403 equippedwith Decision Feedback Equalization (DFE). In a normal operational mode,receiver 403 samples an input data stream from transmitter 402. Thesampled data provides DFE feedback to reduce intersymbol interference(ISI). In a margin-test mode, receiver 403 samples a known input datastream using ranges of sample timing and reference voltages. To preventa collapse of the margins, the DFE feedback path disregards thepotentially erroneous sampled data in favor of an identical version ofthe known input data stream. In-system margin tests can therefore probethe margin without collapsing the margin limits.

Receiver 403 conventionally includes a sampler 405, an optionalclock-and-data recovery (CDR) circuit 410, and a DFE circuit 415. Duringnormal operation, receiver 403 receives a data stream (e.g., a series ofdata symbols) on sampler input terminal Din. Sampler 405 samples thedata stream using a recovered clock RCK from CDR circuit 410 andproduces the resulting sampled data stream on a sampler output terminalDout. DFE circuit 415 stores a plurality of prior data samples and usesthese to condition the input data in the manner discussed above inconnection with FIG. 1. In addition to the conventional components,receiver 403 includes a multiplexer 420, an expected-data source 425,and some comparison logic 430, in this case an exclusive OR (XOR) gate.

During normal operation, a test control signal T to multiplexer 420 isset to a logic zero to connect the output data Dout to the input of DFE415. Thus configured, receiver 403 acts as a conventional DFE-equippedreceiver. In a margin-test mode, however, select signal T is set to alogic one so as to convey an expected data stream from data source 425to the input of DFE 415. Transmitter 402 then supplies known test dataon terminal Din while the expected data is applied to DFE 415. Theexpected data is an identical, time-shifted version of the known dataapplied to input terminal Din, so DFE 415 produces the correct feedbackwithout regard to the output signal Dout. In essence, multiplexer 420provides the feedback path with a first input terminal for sampledoutput data in the operational mode and with a second input terminal forexpected data in the margin-test mode.

The repeated reference herein to “terminal” Din, as opposed to theplural form “terminals,” is for brevity. Receivers may include more thanone data-input terminal, such as those that rely upon differentialsignaling. Likewise, other clock, reference, and signal paths notedherein can be single-ended, differential, etc., as will be evident tothose of skill in the art. The preferred manner in which particular testcircuits and methods are adapted for use with a given receiver willdepend, in part, on the receiver architecture.

A voltage control signal CV on a like-named sampler input terminalalters the reference voltage used by sampler 405 to sample input data. Aclock control signal CC to CDR circuit 410 modifies the timing ofrecovered clock signal RCK. Control signals CV and CC are used in margintesting to explore the voltage and timing margins of receiver 403. Whenthe margin tests reach the margin limits, and thus introduce errors inoutput signal Dout, expected-data source 425 continues to provide thecorrect DFE feedback signal and consequently prevents the margins fromcollapsing in response to the errors. Comparison circuit 430 monitorsthe sampled-data series for errors by comparing the output data with theexpected data from expected-data source 425. In the event of a mismatch,comparison circuit 430 produces a logic one error signal ERR. Asequential storage element (not shown) captures any error signal.Receiver 403 thus facilitates margin testing of DFE-equipped receiverswithout collapsing the margin of interest. (Error signal ERR may or maynot be monitored in the operational mode.)

Expected-data source 425 produces the same data as expected on inputterminal Din. Source 425 can be a register in which is previously storeda known data pattern to be provided during margin testing. Source 425might also be a register that goes through an expected sequence of data,such as a counter or a linear-feedback shift register (LFSR). Regardlessof the source, the expected data presents the expected output data,appropriately timed, to the input of the feedback circuit DFE 415.

FIG. 5 depicts a receiver circuit 500 in accordance with anotherembodiment. Receiver 500 is similar in some ways to receiver 403 of FIG.4, like-numbered elements being the same. Receiver 500 is extended toinclude a second sampler 505 that is substantially identical to, andconsequently mimics the behavior of, sampler 405. The margin tests areperformed on replica sampler 505 so that margin-testing circuitry haslittle or no impact on the performance of receiver 500 in theoperational mode.

Receiver 500 includes a multiplexer 510 connected to a shift register515. A modified clock and data recovery circuit CDR 520 controls thetiming of both samplers 505 and 405. The timing control terminal isomitted for brevity.

Prior to a margin test, test signal T is set to logic zero and thestorage elements within register 515 are loaded with an expected-datasequence. Then, in the test mode, test terminal T is set to logic one sothat shift register 515 feeds its output back to its input viamultiplexer 510. To perform a margin test, sampler 505 samples inputdata Din. Comparison circuit 430 compares the resulting samples with theexpected-data sequence provided by the first storage element in register515. Any difference between the data sampled by the replica sampler 505and the expected sequence from register 515 induces comparison circuit430 to produce a logic one error signal on line ERR. Clocking circuitry,e.g. within CDR 520, can be adapted to control separately the recoveredclock signals RCK1 and RCK2.

FIG. 6 depicts a receiver 600 in accordance with another embodiment.Receiver 600 is similar to the conventional receiver 100 of FIG. 1, butis modified to support improved margin testing.

Receiver 600 includes a sampler 602 that, like sampler 105 of FIG. 1,includes a differential amplifier 115 and a decision circuit 120.Although not shown, sampler 602 includes conventional means of adjustingthe reference voltage and timing to support margin testing. DFE ofreceiver 600 performs conventionally in the operational mode andprovides expected data in the margin-test mode.

Receiver 600 includes a multiplexer 605, a comparison circuit 610, and adual-mode register 615. Multiplexer 605 conveys output signal Dout toregister 615 in the operational mode. Thus configured, receiver 600functions analogously to receiver 100 of FIG. 1. That is, register 615shifts in the output data Dout and employs three bits of historic datato provide ISI-minimizing feedback to sampler 602.

During margin testing, test signal T is set to logic one. In that case,multiplexer 605 provides the output of an XOR gate 620 to the input ofregister 615. The inclusion of XOR gate 620 and the path throughmultiplexer 605 converts register 615 into a linear-feedback shiftregister (LFSR) that provides a pseudo-random but deterministic sequenceof bits to both the input of register 615 and comparison circuit 610.Also during the margin test, the same pseudo-random sequence produced byregister 615 is provided on input terminal Din. This test sequence isapplied one clock cycle ahead of the expected data in flip-flop D1 ofregister 615, so the DFE will reflect the appropriate data regardless ofwhether output data Dout is correct. The timing and reference voltage ofsampler 602 can therefore be adjusted while monitoring output data Doutfor errors without fear of collapsing the margin limits. Comparisoncircuit 610, an exclusive OR gate in this example, flags any mismatchesbetween the output data and the expected data to identify errors.

In the example of FIG. 6, the pseudo-random sequence of test bitsapplied to input terminal Din is assumed to come from an externalsource, such as a conventional tester. The disclosed embodiments canalso be adapted to support built-in self test (BIST) or in-systemtesting. For example, a linked transmitter/receiver pair adapted inaccordance with one embodiment can margin test the intervening link. Inother embodiments, receiver 600 is modified so that register 615 oranother on-chip source provides the input test sequence. In someembodiments, register 615 is extended to include additional storageelements to produce more complex pseudo-random bit sequences. In suchcases, the number of outputs from register 615 to the input of sampler602 can be the same as or different from the number of storage elementsemployed by the LFSR. For additional details regarding LFSRs, see“What's an LFSR,” document no. SCTA036A from Texas Instruments™(12/1996) and the Xilinx™ application note entitled “Efficient ShiftRegisters, LFSR Counters, and Long Pseudo-Random Sequence Generators,”by Peter Alfke, XAPP 052, 7 Jul. 1996 (Version 1.1), both of which areincorporated herein by reference.

FIG. 7 depicts a receiver 700 in accordance with yet another embodiment.FIG. 7 includes a number of elements that are incidental to theinventive margin-testing circuitry, and so are only touched upon brieflyhere. The main components of the margin-testing circuitry arehighlighted using bold outlines to distinguish them from incidentalfeatures. The emphasized components include a pair of conventionalsamplers 705 and 710 receiving input data on the same input terminal,Din, a pair of multiplexers 715 and 720, a pair of shift registers 725and 730, and a data-weighting circuit 735.

In the operational mode, multiplexers 715 and 720 both select their zeroinput. The input data Din captured by samplers 705 and 710 is thusconveyed to respective shift registers 725 and 730. The data in shiftregister 730 is the output data DATA of receiver 700, and is fed back toweighting circuit 735. For equalization feedback, all or a subset of thebits stored in the plurality of storage elements that make up shiftregister 730 are provided to weighting circuit 735. In one embodiment,shift registers 725 and 730 each store twenty bits. Of these, five bitsfrom register 730 are conveyed to weighting circuit 735. The selectedbits and their associated weighting are optimized for a given receiver.For a detailed discussion of methods and circuits for performing suchoptimization, see U.S. application Ser. No. 10/195,129 entitled“Selectable-Tap Equalizer,” by Zerbe et al., filed Jul. 12, 2002, whichis incorporated herein by reference. The details of that referencepertain to the optimization of a number of novel receivers. Themargining methods and circuits disclosed herein may be of use in anysystems that employ historical data to reduce ISI.

Weighting circuit 735 produces a weighted sum of a plurality ofhistorical bits and applies this sum to input terminal Din. This is thesame general function provided by the DFE ladder circuit of FIG. 1,though the manner in which these weighting circuits perform thisfunction differs significantly.

Weighting circuit 735 includes five amplifiers 745[0:4], each of whichreceives a bit from shift register 730. A weight-reference circuit 750provides each amplifier 745 with a reference signal (e.g., a constantcurrent) that determines the weight given to the associated bit. Theoutput terminals of amplifiers 745[0:4] are connected to input terminalDin to provide a weighted sum of five historical data values from shiftregister 730. A current-controlled embodiment of an amplifier 745[i] isdetailed below in connection with FIG. 8.

In the margin-test mode, each of multiplexers 715 and 720 selects its“one” input. The output of sampler 705 is thus conveyed to shiftregister 730 and the output of sampler 710 is conveyed to shift register725. Recall that a function of the margin-test mode is to provideexpected data to the input of the DFE circuitry. In this case, theexpected data is the input data sampled by sampler 705 and captured inshift register 730. A voltage-control signal CV2 and timing controlsignal CT2 allow a tester or test personnel to alter the referencevoltage and received clock RCK2 as necessary to probe the marginboundaries for sampler 710. Similar control signals CV1 and CT1 affordsimilar control over sampler 705 and are set to appropriate levels toensure sampler 705 correctly captures the input data.

During a margin test, erroneous data bits from sampler 710 pass throughshift register 725. Comparison circuit 755 therefore produces alogic-one error signal on line ERR. In this embodiment, it is notnecessary to store expected data in advance or to provide a dedicatedsource of expected data. Instead, the expected data is derived frominput data on terminal Din sampled by sampler 705. The sampler used toproduce output data in the operational mode, sampler 710, is the sameregister subjected to the margin test. Testing the receive circuitry, asopposed to a replica, is advantageous because it provides a moreaccurate reading of the actual receive-circuitry performance. Alsoimportant, sampler 705 can be margined in a normal operating mode,assuming that it has independent timing and voltage control relative tosampler 710. Sampler 705 can also be margin tested and the respectivesample point (voltage and timing) centered in the data eye prior tomargin testing sampler 710.

Receiver 700 of FIG. 7 is an equalizing receiver that generates receiveand equalization clock signals. The following discussion outlinesvarious features of receiver 700. For a more detailed discussion ofsimilar receivers, see the above-incorporated application to Zerbe etal.

In addition to the components discussed above in relation to themargin-testing methods and circuits, receiver 700 includes a CDR circuit756 and an equalizer clock generator 759. Samplers 705 and 710 sampleincoming data signal Din in response to respective receive-clock signalsRCK1 and RCK2, both of which are derived from a reference clock RCLK.The samples taken by sampler 710 are shifted into register 730, wherethey are stored for parallel output via output bus DATA to someapplication logic (not shown) and to CDR circuit 756.

Receive clock signal RCLK includes multiple component clock signals,including a data clock signal and its complement for capturing even andodd phase data samples, and an edge clock signal and a complement edgeclock signal for capturing edge samples (i.e., transitions of the datasignal between successive data eyes). The data and edge samples areshifted into shift registers 725 and 730. Samples in register 730 arethen supplied as parallel words (i.e., a data word and an edge word) toa phase control circuit 761 within CDR circuit 756. Phase controlcircuit 761 compares adjacent data samples (i.e., successively receiveddata samples) within a data word to determine when data signaltransitions have taken place, then compares an intervening edge samplewith the preceding data sample (or succeeding data sample) to determinewhether the edge sample matches the preceding data sample or succeedingdata sample. If the edge sample matches the data sample that precedesthe data signal transition, then the edge clock is deemed to be earlyrelative to the data signal transition. Conversely, if the edge samplematches the data sample that succeeds the data signal transition, thenthe edge clock is deemed to be late relative to the data signaltransition. Depending on whether a majority of such early/latedeterminations indicate an early or late edge clock (i.e., there aremultiple such determinations due to the fact that each edge word/dataword pair includes a sequence of edge and data samples), phase controlcircuit 761 asserts an up signal (UP) or down signal (DN). If there isno early/late majority, neither the up signal nor the down signal isasserted.

Each of a pair of mix logic circuits 763 and 765 receives a set of phasevectors 767 (i.e., clock signals) from a reference loop circuit 769 andrespective timing control signals CT1 and CT2 as noted above. The phasevectors have incrementally offset phase angles within a cycle of areference clock signal. For example, in one embodiment the referenceloop outputs a set of eight phase vectors that are offset from oneanother by 45 degrees (i.e., choosing an arbitrary one of the phasevectors to have a zero degree angle, the remaining seven phase vectorshave phase angles of 45, 90, 135, 180, 225, 270, and 315 degrees). Mixlogic circuits 763 and 765 maintain respective phase count values, eachof which includes a vector-select component to select a phase-adjacentpair of the phase vectors (i.e., phase vectors that bound a phase angleequal to 360°/N, where N is the total number of phase vectors), and aninterpolation component (INT). The interpolation component INT and apair of phase vectors V1 and V2 are conveyed from each of mix logiccircuits 763 and 765 to respective receive-clock mixer circuits 770 and772. Mixer circuits 770 and 772 mix their respective pairs of phasevectors according to the interpolation component INT to generatecomplementary edge clock signals and complementary data clock signalsthat collectively constitute first and second receive-clock signals RCK1and RCK2, which serve as input clocks for samplers 705 and 710,respectively. Timing control signals CT1 and CT2 facilitate independentcontrol of the timing of clock signals RCK1 and RCK2.

Mix logic circuit 765 increments and decrements the phase count value inresponse to assertion of the up and down signals, respectively, therebyshifting the interpolation of the selected pair of phase vectors (or, ifa phase vector boundary is crossed, selecting a new pair of phasevectors) to retard or advance incrementally the phase of the receiveclock signal. For example, when the phase control logic 761 determinesthat the edge clock leads the data transition and asserts the up signal,mix logic 765 increments the phase count, thereby incrementing theinterpolation component INT of the count and causing mixer 772 toincrementally increase the phase offset (retard the phase) ofreceive-clock signal RCK1. At some point, the phase control signaloutput begins to dither between assertion of the up signal and the downsignal, indicating that edge clock components of the receive clocksignal have become phase aligned with the edges in the incoming datasignal. Mix logic 763 and mixer 770 are analogous to mix logic 765 and772, but control the receive clock RCK1 to sampler 705. These redundantcircuits are provided so the receive-clock timing to samplers 705 and710 can be independently adjusted during margin testing.

The equalizer clock generator 759 receives the phase vectors 767 fromthe reference loop 769 and includes mix logic 774 and an equalizer clockmixer 776, which collectively operate in the manner described above inconnection with mix logic 765 and mixer 772. That is, mix logic 774maintains a phase count value that is incrementally adjusted up or downin response to the up and down signals from the phase control circuit761. The mix logic selects a phase-adjacent pair of phase vectors 767based on a vector select component of the phase count. The mix logicthen outputs the selected vectors (V1, V2) and interpolation componentof the phase count (INT) to the equalizer clock mixer 776. Clock mixer776 mixes the selected vectors in accordance with the interpolationcomponent of the phase count to generate the equalizer clock signalEQCLK. The equalizer clock signal, which may include complementarycomponent clock signals, is provided to weighting circuit 735 (oranother type of equalization circuit) to time the output of equalizingsignals onto data input terminal Din.

FIG. 8 depicts an embodiment of a buffer 800 that may be used as one ofamplifiers 745 in weighting circuit 735 of FIG. 7 in an embodiment inwhich the data input Din is a two-terminal port receiving differentialinput signals Din and /Din. Clock signal EQCLK is also a differentialsignal EQCLK and /EQCLK in this embodiment.

Buffer 800 receives one of five differential feedback signals (EQDin[i]and /EQDin[i]) and the differential clock signal (EQCLK and /EQCLK) frommixer 776. Reference circuit 750 provides a reference voltage EQWi thatdetermines the current through buffer 800, and consequently the relativeweight of the selected feedback data bit.

The above-described embodiments are adapted for use in receivers ofvarious types. The embodiment of FIG. 6, for example, is applied to areceiver adapted to receive single-ended input signals, while theembodiments of FIGS. 7 and 8 are applied to receivers adapted to receivecomplementary signals. These examples are not limiting, as these andother embodiments can be applied to receivers adapted to communicatesignals in any of a number of communication schemes, includingpulse-amplitude modulated (PAM) signals (e.g., 2-PAM and 4-PAM), whichmay be used in some embodiments to provide increased data rates.

FIG. 9 depicts a receiver 900 in accordance with another embodiment.Receiver 900 is similar to receiver 700 of FIG. 7, like-identifiedelements being the same or similar. Receiver 900 differs from receiver700 in that receiver 900 omits multiplexer 715 and shift register 725.XOR gate 755 detects errors by comparing the data symbols from samplers705 and 710. As in receiver 700, both samplers 705 and 710 can bemargined in a normal operating mode. The operation of receiver 900 isotherwise similar to that of receiver 700.

Receivers 700 and 900, detailed in connection with respective FIGS. 7and 9, do not require a predetermined pattern of data (i.e., an“expected” data pattern”), and can thus be margined in the presence ofthe data patterns received during normal operation. The ability todetect system margins in system and without disrupting the normal flowof data enables accurate in-system margin test. In addition, receiversso equipped can be adapted to dynamically alter system parameters tomaintain adequate margins.

Margin Mapping (Shmoo Plots)

FIG. 10A depicts a receiver 1000, a simplified version of receiver 900of FIG. 9 used to illustrate margin mapping in accordance with oneembodiment. Receiver 1000 includes two samplers 1005 and 1010, an XORgate 1015, and a “shmoo” circuit 1025. As used herein, a shmoo circuitis used to develop shmoo data, shmoo data is information that representsmargin test results for a given sample point, and a shmoo plot is agraph that represents shmoo data to illustrate how a particular margintest or series of margin tests passes or fails in response to changes inthe reference voltage and reference timing. Samplers 1005 and 1010receive the same input data Din, but have independently adjustablereference voltages RefA and RefB and reference clocks ClkA and ClkB.

FIG. 10B is a diagram 1026 illustrating the relationship between each ofsamplers 1005 and 1010 and a data eye 1030. Each Cartesian coordinate ondiagram 1026 represents a sample coordinate, the Y axis beingrepresentative of sample voltage and the X axis being representative ofsample time. A data point 1035 is centered in data eye 1030 along bothaxes, and thus represents an ideal sample point for sampler 1005.

To perform a margin test, reference voltage RefB and reference clockClkB are adjusted along their respective Y and X axes to sample datasymbols at each coordinate one or more times to probe the boundaries ofeye 1030. Margins are detected when XOR gate 1015 produces a logic one,indicating that sampler 1010 produced different data than sampler 1005.Shmoo circuit 1025 correlates errors with the respective referencevoltage RefB and clock signal ClkB for sampler 1010 and stores theresulting X-Y coordinates. Care should be taken to ensure properclock-domain crossing of the two reference clocks ClkA and ClkB toprevent data samplers 1005 and 1010 from sampling different data eyes(e.g., to prevent respective samplers from sampling different ones oftwo successive data symbols). Signals RefB and ClkB can be interchangedwith respective signals RefA and ClkA in FIG. 10B to margin sampler1010. Methods and circuits for adjusting clock phases and referencevoltages are well known in the art, and are therefore omitted here forbrevity.

FIG. 10C depicts a shmoo plot 1050 graphically depicting an illustrativemargin test in accordance with one embodiment. During margin test,reference voltage RefB and reference clock ClkB are adjusted to sampleincoming data at each voltage/time square (sample point) represented inFIG. 10C. The number of errors encountered over a fixed time is thenrecorded for each sample coordinate. The resulting plot for a givenreceiver will bear a resemblance to plot 1050, though will typically beless uniform than this illustration.

Plot 1050 can be used in a number of ways. Returning to FIG. 10B, forexample, data point 1035 is depicted in the center of eye 1030, an idealcircumstance. Plot 1050 can be used to precisely locate the true centerof eye 1030. Once this center is known, reference voltage RefA andreference clock ClkA can be adjusted as needed to maximize the marginsfor sampler 1005.

Plot 1050 can also be used to establish different margins depending uponthe allowable bit-error rate (BER) for the communication channel ofinterest. Different communication schemes afford different levels oferror tolerance. Communications channels can therefore be optimizedusing margin data gathered in the manner depicted in FIG. 10C. Forexample, an error-intolerant communication scheme might require thezero-error margin, whereas a more tolerant scheme might be afforded thelarger margin associated with a small number of errors per unit time.

Adaptive Margining

Some embodiments detect and maintain margins without storing the shmoodata graphically depicted in FIG. 10C. One or more additional samplerscan be used to probe the margins periodically or dynamically, and thesampler used to obtain the sampled data can be adjusted accordingly. Inone embodiment, for example, the reference voltage and clock of thesampler used to obtain the sampled data are adjusted in response toperceived errors to maintain maximum margins. With reference to FIG.10A, sampler 1010 can periodically probe the high and low voltagemargins and then set reference voltage RefA between them. With referencevoltage RefA thus centered, the process can be repeated, this timeadjusting the phase of reference clock ClkB to detect the timingmargins. The phase of reference clock ClkA can then be aligned in eye1030. In other embodiments, additional samplers can simultaneously probedifferent margins of eye 1030. Dynamic margining systems in accordancewith these embodiments thus automatically account for time-variantsystem parameters (e.g., temperature and supply-voltage).

FIG. 11 details an embodiment of shmoo circuit 1025 of FIG. 10A. Shmoocircuit 1025 includes a pair of flip-flops 1100 and 1105. Flip-flop 1100synchronizes error signal Err with a clock signal Clk. Flip-flop 1105, aones detector, produces a logic-one output signal OUT in response to anylogic ones received from flip-flop 1100. In operation, both flip-flopsare reset to zero and error signal Err is monitored for a desired numberof data samples at a given timing/voltage setting. Flip-flop 1100captures any logic-one error signals Err, and ones detector 1105transitions to logic one and remains there in response to any logic onesfrom flip-flop 1100. A logic one output signal OUT is thereforeindicative of one or more error signals received in the sample period.In other embodiments, flip-flop 1105 is replaced with a counter thatcounts the number of captured errors for a given period. The number andduration of the sample periods can be changed as desired.

FIG. 12 details a double-data-rate (DDR) receiver 1200 in accordancewith another embodiment adapted to accommodate margin shmooing. Receiver1200 includes four data samplers 1205-1208 timed to an odd-phase clockClk_O, four respective flip-flops 1210 timed to an even-phase clockClk_E, three error-detecting XOR gates 1215, a multiplexer 1220,error-capturing logic 1225, and shmoo control logic 1230. An externaltester (not shown) issues test instructions and receives margin-testresults via a test-access port TAP. In another embodiment, the outputsfrom the three flip-flops 1210 following samplers 1205, 1206, and 1207connect directly to corresponding inputs of multiplexer 1220. A singleXOR gate on the output side of multiplexer 1220 then compares theselected sampler output signal with the output from sampler 1208.

As is conventional, DDR receivers receive data on two clock phases: anodd clock phase Clk_O and an even clock phase Clk_E. Receiver 1200represents the portion of a DDR receiver that captures incoming datausing the odd clock phase Clk_O. Signals specific to only one of theclock phases are indicated by the suffix “_E” or “_O” to designate aneven or odd phase, respectively. Samplers 1205, 1206, and 1207 areportions of the “odd” circuitry. Similar samplers are provided for theeven circuitry but are omitted here for brevity. The odd and even clockphases of a DDR high-speed serial input signal can be shmooed separatelyor in parallel.

Receiver 1200 enters a shmoo mode at the direction of the externaltester. Shmoo select signals Shm[1:0] then cause multiplexer 1220 toconnect the output of one of XOR gates 1215 to the input oferror-capturing logic 1225. The following example assumes multiplexer1220 selects error signal Err1 to perform margin tests on sampler 1205.Margin tests for the remaining samplers 1206 and 1207 are identical.

The external tester initiates a shmoo test cycle by issuing a risingedge on terminal Start. In response, control logic 1230 forces a signalRunning high and resets a ones detector 1235 within error-capturinglogic 1225 by asserting a reset signal RST. When signal Start goes low,control logic 1230 enables ones detector 1235 for a specified number ofdata clock cycles—the “shmoo-enable interval”—by asserting an enablesignal EN. When period-select signal PeriodSel is zero, the number ofdata clock cycles in the shmoo-enable interval is 160 (320 symbolperiods). When signal PeriodSel is one, the number of data clock cyclesin the shmoo-enable interval is 128 (256 symbol periods).

The lower-most sampler 1208, in response to control signals from theexternal tester, shmoos the margins for the sampler 1205 selected bymultiplexer 1220. The shmooing process is similar to that describedabove in connection with FIGS. 10A, 10B, and 10C. The process employedby receiver 1200 differs slightly, however, in that receiver 1200 takesadvantage of the presence of even clock Clk_E and flip-flops 1210 toretime the input signals to XOR gates 1215. Even clock Clk_E is 180degrees out of phase with respect to odd clock Clk_O. Clock signal ClkBcan therefore be varied up to 90 degrees forward or backward withrespect to odd clock Clk_O without fear of sampling different datasymbols with the selected sampler 1205 and sampler 1208.

The upper-most XOR gate 1215 produces a logic one if, during theshmoo-enable interval, one or more bits from sampler 1205 mismatches thecorresponding bit from sampler 1208. A flip-flop 1240 captures andconveys this logic one to ones detector 1235. At the end of theshmoo-enable interval, controller 1230 brings signal Running low andholds that state of signal Err_O. A logic one error signal Err_Oindicates to the tester that at least one mismatch occurred during theshmoo-enable interval, whereas a logic zero indicates the absence ofmismatches.

The shmoo interval can be repeated a number of times, each timeadjusting at least one of reference voltage RefD and clock CLKB, toprobe the margins of input data Din. A shmoo plot similar to that ofFIG. 10B can thus be developed for sampler 1205. This process can thenbe repeated for the remaining samplers.

Control logic 1230 does not interfere with the normal operation ofreceiver 1200, so shmooing can be performed for any type of input dataDin. Also advantageous, receiver 1200 allows for the capture of realdata eyes under various operating conditions, and can be used to performin-system margin tests.

Other embodiments repeat the process a number of times for each of anarray of voltage/time data points to derive margin statistics thatrelate the probability of an error for various sample points within agiven data eye. Still other embodiments replace ones detector 1235 witha counter that issues an error sum count for each shmoo-enable interval.

In one embodiment, receiver 1200 samples four-level,pulse-amplitude-modulated (4-PAM) signals presented on terminal Din, inwhich case each of samplers 1205-1207 samples the input data symbolsusing a different reference voltage level. In general, the methods andcircuits described herein can be applied to N-PAM signaling schemes,where N is at least two. Such systems typically include N-1 samplers foreach data input node.

FIG. 13 depicts a receiver 1300 that supports error filtering inaccordance with another embodiment. Receiver 1300 is similar to receiver1000 of FIG. 10A, like-numbered elements being the same or similar.Receiver 1300 differs from receiver 1000 in that receiver 1300 includesdata filter 1305 that allows receiver 1300 to shmoo particular datapatterns. This is a benefit, as a receiver's margin may differ fordifferent data patterns, due to ISI for example. Data filter 1305 allowsreceiver 1300 to perform pattern-specific margin tests to bettercharacterize receiver performance.

Data filter 1305 includes a series of N data registers 1310 that providea sequence of data samples Dout to a pattern-matching circuit 1315. Inthis case N is three, but N may be more or fewer. Data filter 1305 alsoincludes a series of M (e.g., two) error registers 1320 that convey asequence of error samples to an input of an AND gate 1325. AND gate 1325only passes the error signals from registers 1320 as filtered errorsignal ErrFil if pattern-matching circuit 1315 asserts an error-validsignal ErrVal on the other input of AND gate 1325. Pattern-matchingcircuit 1315 asserts signal ErrVal only if the pattern presented byregisters 1310 matches some predetermined pattern or patterns stored inpattern-matching circuit 1315. In one embodiment external test circuitry(not shown) controls the patterns provided by matching circuit 1315.Other embodiments support in-system testing with one or more patternsprovided internally (e.g., on the same semiconductor chip).

Some of the foregoing embodiments employ an additional sampler to probethe margins of a given data input. Some receiver architectures alreadyinclude the requisite additional sampler, to support additionalsignaling modes, for example. Other embodiments may be adapted toinclude one or more additional “monitor” samplers.

While the present invention has been described in connection withspecific embodiments, variations of these embodiments will be obvious tothose of ordinary skill in the art. Moreover, unless otherwise defined,terminals, lines, conductors, and traces that carry a given signal fallunder the umbrella term “node.” In general, the choice of a givendescription of a circuit node is a matter of style, and is not limiting.Likewise, the term “connected” is not limiting unless otherwise defined.Some components are shown directly connected to one another while othersare shown connected via intermediate components. In each instance, themethod of interconnection establishes some desired electricalcommunication between two or more circuit nodes, or terminals. Suchcommunication may often be accomplished using a number of circuitconfigurations, as will be understood by those of skill in the art.Furthermore, only those claims specifically reciting “means for” or“step for” should be construed in the manner required under the sixthparagraph of 35 U.S.C. section 112. Therefore, the spirit and scope ofthe appended claims should not be limited to the foregoing description.

What is claimed is:
 1. A receiver comprising: a data input to receive adata input signal; a first sampler coupled to the data input to samplethe input signal and produce a first sequence of data samples; a secondsampler coupled to the data input to sample the input signal and producea second sequence of data samples; a clock recovery circuit coupled tothe data input, the first sampler, and the second sampler, the clockrecovery circuit to provide a recovered clock signal to at least one ofthe first sampler and the second sampler; a multiplexer having a firstmultiplexer input coupled to the first sampler to receive the firstsequence of data samples, a second multiplexer input coupled to thesecond sampler to receive the second sequence of data samples, and amultiplexer output to convey a selected one of the first and secondsequences of data samples as a selected sequence of data samples; and ashift register coupled to the multiplexer output to convert the selectedsequence of data samples into parallel data.
 2. The receiver of claim 1,wherein the clock recovery circuit provides the first-mentionedrecovered clock signal to the first sampler and a second recovered clocksignal to the second sampler, the first-mentioned and second recoveredclock signals having independently controlled timing.
 3. The receiver ofclaim 1, the first sampler including a first voltage-reference node toreceive a first reference voltage and the second sampler including asecond voltage-reference node to receive a second reference voltageindependent of the first reference voltage.
 4. The receiver of claim 1,further comprising a comparison circuit having a firstcomparison-circuit input coupled to the first sampler to receive thefirst sequence of data samples, a second comparison-circuit inputcoupled to the second sampler to receive the second sequence of datasamples, and a comparison-circuit output to issue error signalsresponsive to mismatches between the first sequence of data samples andthe second sequence of data samples.
 5. The receiver of claim 4, furthercomprising a shmoo circuit coupled to the comparison circuit to receivethe error signals, the shmoo circuit to store information representingmargin test results.
 6. The receiver of claim 1, further comprising anequalizer coupled to the data input.
 7. The receiver of claim 6, whereinthe equalizer is a decision-feedback equalizer.
 8. The receiver of claim7, wherein the decision-feedback equalizer generates feedback from theselected sequence of data samples and applies the feedback to the datainput.
 9. The receiver of claim 1, further comprising a secondmultiplexer having a third multiplexer input coupled to the firstsampler to receive the first sequence of data samples, a fourthmultiplexer input coupled to the second sampler to receive the secondsequence of data samples, and a second multiplexer output to convey aselected one of the first and second sequences of data samples as asecond selected sequence of data samples.
 10. A method for receiving adata input signal applied to a data input, the method comprising:recovering a first clock signal of a first phase and a second clocksignal of a second phase, different from the first phase, from the datainput signal; sampling the data input signal with reference to the firstclock signal to produce a first sequence of data samples; sampling thedata signal with reference to the second clock signal to produce asecond sequence of data samples; comparing the first sequence of datasamples with the second sequence of data samples to issue error signalsresponsive to mismatches between the first sequence of data samples andthe second sequence of data samples; providing the first and secondsequences of data samples to respective inputs of a multiplexer;controlling the multiplexer to convey a selected one of the first andsecond sequences of data samples as a selected sequence of data samples;and converting the selected sequence of data samples into parallel data.11. The method of claim 10, further comprising varying the first phaserelative to the second phase while sampling the data input signals withreference to the first clock signal and the second clock signal andcorrelating the differences between the first and second phases with theerror signals.
 12. The method of claim 11, further comprising plottingthe error signals with respect to the differences.
 13. The method ofclaim 12, the plotting comprising generating a shmoo plot.
 14. Themethod of claim 10, further comprising varying a first reference voltageagainst which to sample the data input signal to produce the firstsequence of data samples relative to a second reference voltage againstwhich to sample the data input signal to produce the second sequence ofdata samples and correlating the differences between the first andsecond reference voltages with the error signals.
 15. The method ofclaim 14, further comprising plotting the error signals with respect tothe differences.
 16. The method of claim 15, the plotting comprisinggenerating a shmoo plot.
 17. The method of claim 10, further comprisingequalizing the data input signal.
 18. The method of claim 17, whereinequalizing the data input signal comprising feeding back data samplesfrom at least one of the first sequence of data samples and the secondsequence of data samples.